Urea is produced from ammonia and carbon dioxide. Today's urea production involves relatively clean processes, particularly low in the emission of urea dust and ammonia. However, besides the chemical synthesis of urea, the production of urea on a commercial scale requires that the urea be presented in a suitable solid, particulate form. To this end, urea production involves a finishing step in which a urea melt is brought into the desired particulate form, generally involving any one of prilling, granulation, and pelletizing.
Prilling used to be the most common method, in which the urea melt is distributed, as droplets, in a prilling tower and whereby the droplets solidify as they fall down. However, the end-product is often desired to have a larger diameter and higher crushing strength than the one resulting from the prilling technique. These drawbacks led to the development of the fluidized bed granulation technique, where the urea melt is sprayed on granules that grow in size as the process continues. Prior to the injection in the granulator, formaldehyde is added to prevent caking and to increase the strength of the end-product.
In order to remove the energy released during crystallization, large amounts of cooling air are fed to the granulation unit. The air that leaves the finishing section contains, inter alia, urea dust. With a view to increased demand for urea production, and increasing legal and environmental requirements as to reduce the level of emissions, it is desired that the urea dust is removed, and according to ever increasing standards.
Over the past several decades the control of air pollution has become a priority concern of society. Many countries have developed highly elaborate regulatory programs aimed at requiring factories, and other major sources of air pollution, to install the best available control technology (BACT) for removing contaminants from gaseous effluent streams released into the atmosphere. The standards for air pollution control are becoming increasingly stringent, so that there is a constant demand for ever more effective pollution control technologies. In addition, the operating costs of running pollution control equipment can be substantial, and so there is also a constant demand for more efficient technologies.
The removal of urea dust is challenging per se, since the amounts of off-gas (mainly air) are enormous, whilst the concentration of urea dust is low. A typical airstream is of the order of 750 000 Nm3/h. A typical concentration of urea dust therein is about 2 wt. %. Further, part of the urea dust is of a submicron size. Satisfying current standards implies the need to remove a major part of this submicron dust.
A further problem is that the large amounts of air needed in urea finishing, results in this part of the production process being a relatively costly effort due to the need for very large extractor fans having a large electricity consumption. Particularly, when the air is subjected to scrubbing in order to reduce the emission of urea dust, and specifically a major part of the submicron dust, into the atmosphere, a relatively large amount of energy is simply lost in the process, as a result of the inevitable pressure drop in the scrubbing device.
One well known type of device for removing contaminants from a gaseous effluent stream is a venturi scrubber. Venturi scrubbers are generally recognized as having the highest fine particle collection efficiency of available scrubbing devices. In a venturi scrubber the effluent gas is forced or drawn through a venturi tube having a narrow “throat” portion. As the air moves through the throat it is accelerated to a high velocity. A scrubbing liquid in the form of droplets, typically of water, is added to the venturi, usually at the throat, and enters the gas flow. The water droplets used are generally many orders of magnitude larger than the contaminant particles to be collected and, as a consequence, accelerate at a different rate through the venturi. The differential acceleration causes interactions between the water droplets and the contaminant particles, such that the contaminant particles are collected by the water droplets. The collection mechanisms involve, primarily, collisions between the particles and the droplets and diffusion of particles to the surface of the droplets. In either case, the particles are captured by the droplets. Depending on the size of the contaminant particles, one or the other of these mechanisms may predominate, with diffusion being the predominant collection mechanism for very small particles, and collision or interception being the predominant mechanism for larger particles. A venturi scrubber can also be efficient at collecting highly soluble gaseous compounds by diffusion. A detailed description of these scrubbing mechanisms is discussed in Chapter 9 of Air Pollution Control Theory, M. Crawford, (McGraw-Hill 1976).
One of the main characteristics of this type of scrubber, is that it causes a larger pressure drop than other scrubbers, the estimated pressure drop required to reach the desired high collection efficiency being about 100 mbar. yet, in view of its suitability for the removal of submicron particles (such as urea dust), it would be desired to make use of a venturi scrubber. It will be understood that using a venturi-type scrubbing device presents a further desire to reduce the inevitable loss of energy associated therewith.
Some background references refer to the use of venturi scrubbing in urea finishing.
FR 2 600 553 relates to removing dust from gases, such as form from urea prilling. The method as described includes subjecting the gas to prewashing, by spraying a liquid into the gas stream, prior to venturi scrubbing. The purpose of the pre-washing step is that no additional scrubbing liquid is added, which would lead to a low pressure drop. I.e., the washing liquid is applied in such a way as to produce droplets that are of a sufficiently large size to wash out small particles.
EP 514 902 relates to a method for the removal of urea dust from the off-gas of a finishing section of a urea production plant. Water is added to act with a venturi scrubber, flowing down by gravity along the walls of the venturi as a film. The gas flowing upward is atomizing the film thereby forming a scrubbing liquid, i.e. with the purpose to form liquid droplets that interact with ammonia, and optionally urea dust, to be removed.
In fact, most venturi scrubbers in use today are “self-atomizing”, i.e., the droplets are formed by allowing a liquid to flow into the throat of the venturi where it is atomized by the gas flow. While very simple to implement, this method is not able to produce droplets of very small median diameter.
The primary methods utilized in improving the collection efficiency of a venturi scrubber have been to decrease the size of the throat or to increase the overall rate at which gas flows through the system. Both of these methods increase the differential velocities between the contaminant particles and liquid droplets as they pass through the throat of the venturi. This causes more interactions between particles and droplets to occur, thereby improving contaminant removal. However, increasing the collection efficiency in this manner comes at a cost of significantly higher energy input into the system, thereby resulting in higher operating costs. The extra energy is expended due either to the increased overall flow resistance attributable to the reduced throat diameter, or to the increased overall flow rate through the venturi. In either case, the pressure drop across the venturi is increased and greater pumping capacity is required. Accordingly, heretofore, efforts to increase the fine particle collection efficiency of a venturi scrubber have involved substantial increased energy input into the system.
Of particular concern to those in the field of air pollution control is the collection of “optically active” particles. As used herein, the term “optically active particles” should be understood to mean particles having a diameter in the range of approximately 0.1 to 1.0 microns. In an effort to control these particles, the EPA has recently reduced the “PM 2.5 standards” for the emissions of particles less than 2.5 microns. These and smaller particles are difficult to collect in conventional venturi scrubbers due to their small size. Nonetheless, particles in this size range are currently responsible for the measured emissions.
What is desired is an apparatus and method that permits the efficient and economical scrubbing of fine particles from a large gas flow using a cleansing liquid in a venturi scrubber. Specific needs include reduced scrubbing liquid pumping requirements, lower pressure drop across the venturi, improved scrubber performance, and better control of the pressure drop across the venturi scrubber.
It is now desired to provide a method for treating the off-gas of a urea finishing section in such a way as to effectively remove urea dust. It is further desired to provide a method by which this removal is improved. And, moreover, it is desired to achieve this in a process of improved energy efficiency.
Still another object of the present invention is to provide an air pollution control system which is capable of compensating for variations in the flow through the system.